In the computing industry, laser printers have quickly become the standard for producing high quality, hard copy results. Image resolution is one of the characteristics that determines the quality of the printer output. For example, a 600 dot per inch (dpi) resolution laser printer has twice the resolution over a 300 dpi printer. Similarly, a 1200 dpi printer has twice the resolution over a 600 dpi printer. A "dot" or "spot" represents a laser pulse and is a basic building block for creating images in a laser printer. Essentially, a dot is an electrical charge imprint produced on a transfer medium by a laser diode for attracting toner in creating the image.
Generally, the more dots per inch, the better the resolution because the dots are closer together and thereby define sharper edges for the images produced. However, an increase in dpi usually increases a printer's base cost due to the added memory and improved hardware required to satisfy the increased dpi. Accordingly, the balancing of tradeoffs between cost and performance is an ongoing issue.
One conventional method of increasing a printer's resolution has been to pulse the laser diode more rapidly as the beam moves across the photoconductive drum in a conventional scan path. However, this method increases the dots per inch in the scan direction only. Although this method provides a relatively enhanced resolution, it does not provide resolution enhancement in both the x and y axis directions relative to the drum. Namely, more dots per inch are achieved in the scan direction but not in the process direction.
The development of gray scale technology has provided an alternate method for effectively increasing resolution. Gray scale selectively reduces the dot size in a printer to provide a better "fit" of the dots in an image. Dots are reduced from about 20% to 100% by controlling the pulse width modulation (PWM) of the laser diode. PWM is the modification of the duty cycle of the video (laser) signal wave form within a unit amount of time and has the apparent effect of changing the signal intensity. The duty cycle is the percent of time the signal is in an active state within the specified unit amount of time.
Although a 300 dpi gray scale printer provides one means for selectively increasing resolution over a 300 dpi binary (non gray scale) printer, it does not provide the enhanced resolution equal to a 600 dpi gray scale printer. Similarly, a 600 dpi gray scale printer does not provide an enhanced resolution equal to a 1200 dpi gray scale printer, and so forth. However, recent studies have shown that photo quality images are achievable at densities of 300 dpi gray scale. The use of 600 dpi, or greater, is required only where extremely sharp edges are required, such as in text or graphics. This suggests that a further solution (other than gray scale) for providing selectively increased resolution may satisfy many enhanced resolution printing needs.
Laser printers operate by scanning a laser beam horizontally across the photosensitive, electrically charged drum. If the laser beam is modulated, variations in charge will ultimately be translated to proportionate amounts of toner deposited on a sheet of paper (such as discussed with gray scale). However, since laser printers are designed to run very fast, this architecture has proven to be extremely sensitive to variations in drum speed. These variations appear on the printed page as increased or decreased spacing between lines and visually appear as "bands". This undesirable effect is called banding. Banding is a particularly severe problem for faster laser printers which are printing gray scale images, such as photographs. Research has shown that the most severe banding effect occurs at intermediate levels of gray.
The principle cause of banding is due to gear noise, although stepper motor frequencies and scanner frequency variations also contribute slightly to this problem. Gear noise results from imperfect spacing of gear teeth, variances in flexing of gear teeth as forces are transferred from one gear to the next, and other intrinsic variations in gear force transfer. The stepper motor contributes to banding because as it drives the gear array in a laser printer it may have slight variations in angular velocity due to the multiple magnet positions for each step. The scanner assembly includes a rotating multi-sided mirror, the laser diode, and associated optics.
Since new printer products are consistently designed to print faster, the problem of banding is likely to worsen. Conventionally, attempts at reducing banding effects have been focused on mechanical fixes related to gear noise, the stepper motor, and/or the scanner assembly. For example, mechanical fixes may involve gears with helical drive or gears made of better materials or with greater precision, but these generally add significantly more expense to the final product. Furthermore, these approaches do not address the root cause of the banding problem, that is, the open loop (no feedback) nature of how the drum is rotated. Namely, the drum is driven by a constant speed motor drive system, and no feedback from any source is used to modify the motor speed or to correct some of the previously mentioned contributions to banding.
Accordingly, given the forgoing backgrounds relating to printer resolutions and banding, objects of the present invention are to provide a new system and method for (1) increasing resolution in a laser printer without increasing the hardware and memory costs conventionally associated with enhanced resolution printers, and (2) reducing the visual impact of banding.